A common type of hearing protection comprises two caps which are connected by a headband. The headband is adapted to be placed over the head of the user and the two caps are adapted to be arranged one on each side of the user's head so as to enclose both external ears of the user. The cap in turn comprises a cap shell, which usually is made of plastic, a sealing ring, which is adapted to fit the head of the user, and some kind of sound-absorbing material arranged inside the cap. It is also common for a so-called bottom plate to be arranged, to which the sealing ring is attached and which in turn is attached to the cap shell. Thus the bottom plate is positioned between the cap shell and the sealing ring. The bottom plate also usually acts to keep the sound absorbing material in place in the cap. As a rule, the sealing ring and bottom plate are integrated with each other, that is they are made as one unit.
The function of the hearing protection is to absorb noise from noise sources in the surroundings so that the noise does not reach the user's ears without first being dampened to acceptable levels. However, noise at certain frequencies or in certain frequency ranges may create resonance phenomena in the caps of the hearing protection, which significantly impairs the dampening of such noise.
It is known that a greater weight of the cap gives better attenuation of incoming noise, especially in the low frequency sound range, i.e. below about 1 kHz. To increase the weight of the cap, it is possible to increase the thickness of material, that is the thickness of the cap shell. Increasing the thickness “outwards” gives the drawback that the hearing protection will be larger and more unwieldy. Increasing the thickness “inwards” gives the drawback that the sound attenuation can even be impaired since the inner volume will be smaller, and a high degree of absorption requires not only weight but also a large inner volume of the cap—the relative variation in pressure of the air inside the cap will be smaller when the air volume is larger.
U.S. Pat. No. 6,353,938 B1 discloses a hearing protector with an outer cap shell of a rigid plastic material, to which a filler in the form of metal powder is added so as to increase the weight of the cap. The possibility of increasing the weight in this manner is, however, limited since there is an upper limit of how much metal can be admixed to the plastic in view of tool wear, strength properties etc. Furthermore an increase of the weight of the cap shell affects low frequency sound the most. Thus there remains the problem of damping resonance at higher frequencies, i.e. above about 1 kHz.
U.S. Pat. No. 4,658,931 discloses a hearing protector with two caps which each comprise an outer and an inner cap shell and, arranged therebetween, an evacuated cavity for attenuation of sound, which is free of transmission medium. The document mentions that a resonance frequency of one cap shell can be separated from a resonance frequency of the other cap shell by the weight of one cap shell being increased, for instance by adding extra weights inside the closed cavity. In this manner, the resonance frequency of this one cap shell is reduced, whereby the resonance frequencies of the two shells are separated. Just like in U.S. Pat. No. 6,353,938 B1, it is thus shown how sound attenuation can be improved by an increased weight of cap shells, which has the greatest effect on low frequency sound, but no solution is provided to the problem of improving sound attenuation in the higher frequency range.